Field of the Invention
The present invention relates to a method for preparing a cellulose suspension in organic solvents such as an aqueous NMMO solution, a substantially dry cellulosic raw material being mixed with the suspending agent in a slurrying unit, whereby a cellulose suspension is obtained, the cellulose suspension being squeezed to a cellulose content of 9-15%, and the moist cellulose being fed to a dissolution unit. Furthermore, the invention relates to a method for producing molded cellulosic bodies.
Description of Related Art
The production of molded cellulosic bodies such as fibers, filaments, foils by dissolving cellulosic raw materials and subsequent precipitation has been known for a long time. An example of such a method is the so-called amine oxide method wherein a cellulosic raw material is dissolved in a solvent, substantially consisting of an amine oxide, preferably N-methylmorpholine N-oxide (NMMO), and water, and is subsequently precipitated in the desired shape in an aqueous precipitation bath.
U.S. Pat. No. 4,246,221 describes an amine oxide method for the preparation of spinnable cellulose solutions, which as a base material uses, among other things, a mixture of cellulose in liquid aqueous N-methylmorpholine N-oxide (NMMO). According to this method a suspension of comminuted cellulose in the aqueous amine oxide solution is prepared in a discontinuously operating mixing apparatus and the mixture is heated simultaneously under reduced pressure, with water being removed and a first solution being prepared which, following filtration and postprocessing in an extruder, is transformed into a moldable solution. According to Example I, the mixing appliance used to prepare the cellulose suspension is a conventional double arm mixer. In this mixer, a suspension having a pulp consistency of about 20% by mass of cellulose is prepared. However, the above-described method has the disadvantage that it is discontinuous and difficult to implement at an industrial scale.
Furthermore, in U.S. Pat. No. 4,246,221 it is recommended, for the particularly gentle and rapid dissolution of the cellulose, to employ it and the (solid) amine oxide hydrate in a ground state. This is disadvantageous, however, as the cellulose will be damaged by the grinding process, e.g., if local overheating occurs. Furthermore, it is disadvantageous to employ a solid mixture of comminuted cellulose and comminuted amine oxide as a base material for the preparation of the cellulose solution, as it is known from EP-A-0 419 356 that a suspension of cellulose in an aqueous amine oxide can, with the help of thin film technology, be transformed into the moldable cellulose solution faster, gentler, and better.
U.S. Pat. No. 4,416,698 also recommends to those skilled in the art to grind the cellulose, namely, down to a particle size of less than 0.5 mm.
According to DD-A-226 573, the solution is prepared based on an NMMO-containing cellulose suspension having a low pulp consistency of no more than 2.5% by mass of cellulose. The NMMO concentration in the pure suspending medium is to be approx. 70% by weight. This cellulose suspension is homogenized in an agitator vessel. Subsequently, the pulp consistency is increased to 12.5% by mass by centrifuging or squeezing, the water content in the suspension is reduced to 10-15% by mass (related to NMMO), and finally the suspension is transformed into a clear solution in an extruder with a degassing zone at temperatures between 75 and 120° C. The method according to said DD-A-226 573 has, among other things, the disadvantage that, following the homogenization, the pulp consistency must be increased from 2.5% by mass to 12.5% by mass before it is possible to proceed to actually preparing the solution. Not only does this require a very large quantity of suspending medium that needs to be kept circulating, but also a substantial effort in order to separate the excess suspending medium again. While DD-A-226 573 mentions the method of squeezing, it does not provide any answers how this can be implemented in practice, especially at an industrial scale. For example, it is not disclosed to those skilled in the art how a consistent degree of squeezing, which is a prerequisite for a constant composition of the solution, can be obtained.
From WO 95/11261 of the applicant a method is known according to which (1) precomminuted cellulosic materials are introduced into an aqueous solution of a tertiary amine oxide in order to prepare a first suspension having a pulp consistency of at least 10% by mass of dry cellulose, (2) the first suspension is subjected to grinding, whereby a second suspension is obtained, and (3) the second suspension is transformed into the moldable cellulose solution by applying heat under reduced pressure. Grinding is used to further comminute occasional remaining pulp particles and split them into individual fibers. The grinding devices proposed are conventional high consistency mixers, dispersers, and refiners.
From WO 94/28217 a method for the preparation of a premixture of comminuted cellulose and aqueous amine oxide is known, wherefrom a moldable cellulose solution can be prepared. The base material used is pulp in roll form that is first precomminuted in a cutting machine (shredder). It is pointed out in WO 94/28217 that, when the pulp is cut, it should be ensured that it is compressed as little as possible at the cut edges, as this would otherwise make subsequent mixing with the aqueous amine oxide solution more difficult. For this purpose, a special cutting machine is recommended in which pulp pieces of a size of typically no more than 15 cm2 are produced. As a side product of the cutting process, however, considerable quantities of pulp dust are generated. After cutting in the cutting machine, the precomminuted pulp is comminuted further in a fan having propeller blades and is transported by means of air to a screen where the precomminuted pulp is separated from the air flow. The screen separates all pulp particles having a size of at least 2.54 mm. The remaining air flow, however, still contains a significant portion of dust with particle sizes of less than 2.54 mm. To avoid losing this pulp, it is collected in a filtering device and finally united again with the larger pulp particles. To prepare the suspension, the comminuted cellulose and the amine oxide solution are introduced into a horizontally supported, cylindrical mixing chamber that has a rotor with axially spaced agitator members. The mixture is stirred in the mixing chamber by rotating the rotor at a speed of 40 to 80 revolutions per minute. Preferably, quickly rotating refiner blades are also provided at the wall of the cylindrical mixing chamber and used to comminute the pulp particles. The combined action of the agitator paddles and the quickly rotating refiner blades eventually yields a homogenous mixture that includes up to about 13% by mass of cellulose, related to the mass of the suspension. Nevertheless, this previously known method has the disadvantage that it is technically complex and very time-consuming to eventually prepare the homogenous suspension from the pulp in roll form. What is more, the previously known method is discontinuous and takes more than 20 minutes per batch just for mixing.
EP 0853642 notes, first of all, that conventional pulpers, as employed in various configurations in viscose technology or in the papermaking industry, cannot be used satisfactorily for the present objective, as they are said to permit the preparation of cellulose suspensions having a pulp consistency of no more than only 11.5%. The same document also notes that, due to the high degree of swelling of the pulp in 60-78% aqueous NMMO at the required processing temperature of 60-90° C., the flowability of the suspension would decrease to such a great extent that the pulp/NMMO/water mixture is no longer subject to sufficient shear and mixing throughout the entire mixed materials. In Comparison Example 5 contained therein, when using a conventional pulper, a 74% NMMO solution at 70-80° C., and the stepwise addition of pulp in sheet form without precomminution, it is established that the process had to be discontinued after about 12 minutes and after reaching a pulp consistency of 11.57, as no more increase of the pulp consistency was possible because no mixing was taking place in the peripheral zones and thus no suspension was obtained.
Instead, EP 0853642 proposes a method for preparing a homogenous suspension of cellulose in an aqueous solution of a tertiary amine oxide, wherein pulp is mixed with the aqueous solution of the tertiary amine oxide in a mixing apparatus that has a container to receive the suspension and a mixing tool, and which is characterized in that a mixing apparatus is employed whose container rotates during mixing. This should make it possible to prepare a cellulose suspension that can be fed directly into a dissolution stage. The aqueous solution of the tertiary amine oxide employed in this method contains the amine oxide, preferably NMMO, at between 60 and 82% by mass. The method is conveniently executed at a temperature between 60 and 90° C. This made it possible to prepare cellulose suspensions having a pulp consistency of more than 13% and, on top of that, to even use, for this purpose, a concentrated amine oxide solution having 78% by mass of NMMO. It was also noted, however, that these effects apparently cannot be obtained when using a mixer with an immovable, i.e., a non-rotating container and a rotating mixing tool.
WO 2005/113869 A1 finally describes a method and an apparatus for preparing a cellulose solution that can be extruded into endless molded bodies, wherein, at first, a cellulose suspension is prepared from cellulose and water in a pulper, which is subsequently squeezed with a squeezing means. Following squeezing, a tertiary amine oxide, particularly N-methylmorpholine N-oxide, is added as a solvent to the cellulose suspension in order to prepare a cellulose solution. This method has the disadvantage that large quantities of water are introduced into the cellulose, which must subsequently, when preparing the solution, be evaporated again by means of using a large amount of energy.